1. Field
This disclosure relates to a platform for stereoscopy using hand held film/video camera stabilizers.
2. General Background and State of the Art
Humans (and many animals) use binocular vision to view the environment in three dimensions. Binocular vision is both a visual and an analytical system. The brain perceives distance and speed based, in part, on triangulating visual information that the retinas of the laterally separated, forward facing eyes receive. Because both eyes face forward and are about 6½ cm-7 cm (2½ in.-3 in.) apart, their respective fields of view overlap, with each eye perceiving a slightly different perspective of the same area. Focusing on objects closer to our eyes, causes the eyes to rotate towards each other. Focusing on more distant objects cause the eyes to rotate towards a more parallel view. The angle between the lines of sight of each eye is commonly termed the convergence angle. The convergence angle is considered higher when viewing closer objects and lower when viewing distance objects. The convergence angle may be essentially zero, indicating essentially parallel lines of sight, when viewing objects at great distance. The eyes also change focus when changing between viewing distant and near objects. The brain process information about the convergence angle (binocular) and focus (monocular) to perceive distance and velocity
Three-dimensional imaging, also known as stereographic imaging, dates at least as far back as 1838. Historically, stereographic cameras commonly include two lenses spaced laterally apart a similar distance as an average human's eyes, approximately 6½ cm-7 cm. The effective distance of the lenses from each other is known as the interocular distance. The interocular distance has a strong effect on the apparent depth of a stereographic image. Increasing the interocular spacing increases the apparent depth of a stereographic image. Decreasing the interocular spacing decreases the apparent depth of a stereographic image.
To perceive images in three dimensions, a first image to be seen only by the left eye and a second image to be seen only by the right eye are projected on a screen or monitor. Differences, or disparity, between the two images may provide an illusion of depth so that the two images having disparity may be perceived as three-dimensional.
Viewers may not perceive two images or portions of two images exhibiting excessive disparity as three-dimensional but simply as two overlapping two-dimensional images. The amount of disparity that a viewer can accommodate, commonly called the disparity limit, varies among viewers. The disparity limit is also known to vary with image content, such as the size of an object, the proximity of objects within an image, the color of objects, and the rate of motion of objects within the image. The disparity limit, expressed as the angle between the lines of sight of the viewer's eyes, may be about 12-15 minutes of arc for typical stereoscopic images.
A variety of techniques, including polarization, filters, glasses, projectors, and shutters have been used to restrict each eye to viewing only the appropriate image.
One approach to displaying stereographic images is to form the left-eye image on a viewing screen using light having a first polarization state and to form the right-eye image on the same viewing screen using light having a second polarization state orthogonal to the first polarization state. The images may then be viewed using glasses with polarizing lenses such that the left eye only receives light of the first polarization state and the right eye only receives light of a second polarization state. Stereoscopic displays of this type typically project the two polarized images onto a common projection screen. This technique has been used to present 3-D movies.
A second approach to displaying stereographic images is to form the left-eye and right-eye images alternately on a common viewing screen at a high rate. The images may then be viewed using shutter glasses that alternately occult either the right or left eye in synchronism with the alternating images.
Two cameras or other image acquisition devices are required to capture the left and right images. Examples of image acquisition devices include charged coupled devices (CCD), complementary metal-oxide semiconductor (CMOS) devices, film and other devices. Image acquisition devices may acquire visual information singly or in sequence. The image acquisition devices are part of more complex cameras that include a lens, control circuitry, image storage (on or off the camera) and other structure for using the camera. The lens can focus, zoom and change aperture (f-stop) usually under control of electronically controlled motors.
The two cameras can mount next to each other. application Ser. No. 11/422,048, filed Jun. 2, 2006, now U.S. Pat. No. 7,643,748 B2, discloses a platform for mounting two adjacent cameras on separate convergence plates. The application is incorporated by reference. Rotating or displacing the convergence plates changes the angle of the cameras relative to each other and can more the cameras toward or away from each other. For example, when each convergence plate is rotated in a direction moving the front (lens) of each camera toward each other, the angle of convergence increases. The resulting effect is one of shifting stereoscopic depth of field from a more distant to a closer position. Pivoting each convergence plate in the opposite direction produces the opposite effect.
Pivoting both plates may be unnecessary because pivoting one plate changes the convergence angle.
The convergence plates also can move relative to each other with or without a corresponding pivoting. Moving one plate away from the other plate increases the interocular distance between the cameras' image acquisition devices. The greater interocular distance increases the stereoscopic depth of field. People do not see stereoscopically beyond about 20 ft. (6 m). For example, people attending a basketball game are usually too far from play to view the action stereoscopically. However, a video creator may want a stereoscopic effect from the more distant action. Increasing the interocular distance can yield stereoscopic effect for more distant action. Choosing the interocular distance and convergence angles are often creative and artistic choices.
Camera lenses usually have a wider diameter than a human eye has. Even with the lenses touching each other, the axes of the lenses may be farther apart than human eye spacing. For natural stereoscopic viewing, having to the lens spacing approximately equal to human eye spacing (6½-7 cm) can be important. To overcome this potential problem, the two cameras can be mounted spaced and perpendicular to each other. Images enter one camera through a half-silvered mirror, and the other images reflect from the mirror to the other camera. Because the cameras are spaced apart in perpendicular planes, their lenses do not contact each other. Consequently, distance between the lenses' axes can be zero or at any desired spacing subject to the dimensions of the platform.
A Steadicam®, which is sold by The Tiffen Co. of Hauppauge, N.Y., is one of several models of similar camera support devices. They allow camera operators to support a camera and move while filming scene. See for example, Brown, U.S. Pat. No. 4,474,439 (1984). Using the word “filming” does not imply that photographic film is used. The word applies to all types of image capture. A Steadicam comprises a sled pole with camera (here two cameras) mounted on the upper portion of the pole and a ballast mounted at the lower end of the pole. The ballast may include batteries for supplying electrical power to the camera and other equipment. The camera/ballast positions can be reversed for recording from low, near-the-ground positions. A monitor also may mount on the pole at a convenient viewing position and angle for the operator. The weight distribution increases the systems moment of inertia to maintain the camera steady and eliminate or minimize small camera movements.
To allow the operator to move the camera vertically, the sled pole connects to a support arm such as that shown in U.S. Pat. No. 4,208,028 (1980) and U.S. Pat. No. 5,360,196 (1994). They teach linkages with pairs of arm sections forming parallelograms, each with a link on the operator end and a link on the pole end. The linkage maintains the pole vertical as the pole moves vertically. Springs on the linkage compensate for the weight of the pole and its equipment. With cameras, ballast, monitor and other equipment, the camera support can be quite heavy. Moreover, the linkage isolates the pole and equipment from the operator to limit transmission of his or her vertical movements during walking or running to the camera. This is sometimes called “buoyancy.”
The pole has a gimbal mount positioned at the center of gravity of the pole and the equipment mounted on the pole. The gimbal attaches to a support garment worn by the operator directly or indirectly through the linkage. The gimbal is fixed at a vertical positioned along the pole to be at the center of gravity. Insofar as the weight of the camera(s) and their support structure may not be along the axis of the pole, the ballast and other equipment may be positioned toward or away from the pole to locate the center of gravity along the pole. Likewise, the ballast and other equipment may have to be positioned relative to each other for similar reasons. This is especially true because any monitor may have to be positioned so the operator can have a clear view of it.
As the camera operator walks or runs through the scene, he or she steers or guides the camera through the scene. The gimbal mount allows the cameras to pivot forward and back, side to side and in between.
Before filming each scene begins (i.e., before each “take”), the operator starts with the camera support balanced about the center of gravity at the gimbal such that there is no force tending to rotate the pole about the center of gravity.
With non-stereoscopic filming, the system's weight distribution remains constant. However, during stereoscopic filming, one or both cameras may move laterally to change interocular displacement or may pivot to change the convergence angle. Even small movement change the center of gravity of the system. When that occurs, the operator must try to overcome the changing weight distribution and force the pole back to vertical while still guiding the camera during filming. This is difficult for the operator. For larger camera lateral movements or changes in convergence angles, the operator may be unable to compensate for the change in weight distribution.
Even when filming a one-dimensional scene, preventing the camera from tilting is important so that objects will not lean when the image is projected. Preventing tilting of two-camera stereoscopic is important to maintain the stereoscopic effect.
Throughout the following detailed description, elements appear in more than one drawing, but the drawings do not always repeat every reference numeral in every figure.